Where to find the serial number

Water heater: the rating label on the side of the tank, near the EnergyGuide sticker. On a tankless unit it is on the side or bottom of the cabinet.
Furnace: inside the front panel or on the side of the cabinet.
AC, heat pump or mini-split: the plate on the outdoor unit, usually near the line connections.
Use the line marked Serial No. or S/N, not the model number. If the plate prints a manufacture date, use that.

Brands we decode

Water heaters

A.O. Smith · American Water Heater · U.S. Craftmaster · Bradford White · State · Reliance · Kenmore · Whirlpool · Rheem · Ruud · Richmond · GE · Marathon · Rinnai · Navien · Noritz · Takagi · Bosch (tankless water heater) · Lochinvar · GSW / John Wood · Paloma · HTP / Westinghouse / American Standard (water heaters) · Giant · EcoSmart / Eemax · Stiebel Eltron

Furnaces, AC and heat pumps

Goodman · Amana (heating and cooling) · Daikin · Carrier · Bryant · Payne · Day & Night · Heil / Tempstar / Comfortmaker / Arcoaire / KeepRite · Lennox · Armstrong Air / AirEase / Concord / Allied · Ducane · Aire-Flo · Trane · American Standard (heating and cooling) · York · Luxaire · Coleman / Evcon · Fraser-Johnston / Champion / Guardian · WeatherKing · Nordyne / Intertherm / Miller / Maytag / Frigidaire / Tappan / Westinghouse / Kelvinator / Gibson / Broan · Comfort-Aire / Heat Controller / Century

Mini-splits and geothermal

Mitsubishi Electric · LG · Fujitsu · Gree · Bosch (heat pumps and geothermal) · ClimateMaster · WaterFurnace

Is it time to replace it?

Should I replace my water heater? · Should I replace my furnace? · Should I replace my air conditioner?

How long does a water heater last? · How long does a tankless water heater last? · How long does a heat pump water heater last? · How long does a furnace last? · How long does a central air conditioner last? · How long does a heat pump last? · How long does a ductless mini-split last? · How long does a geothermal heat pump last?

Parts that help it last

Anode rod The sacrificial rod that rusts instead of the tank. Checking it every few years is the cheapest way to add life.
Flush kit A hose and fittings for draining sediment from the bottom of the tank once a year.
Expansion tank Soaks up pressure swings in a closed plumbing system. Ask a plumber whether yours needs one.
Leak alarm A small alarm that goes on the floor beside the heater and warns you before a slow leak becomes a flood.
Furnace filters Check the size printed on the old filter and buy the same. A clogged filter is the most common cause of avoidable furnace trouble.

Check your model and size before you buy. Amazon links are affiliate links: as an Amazon Associate I earn from qualifying purchases.

Keep a record of your home

The age you just looked up is the kind of fact that gets lost. A written record of serial numbers, install dates, repairs and who did the work saves the next owner, inspector or insurer from guessing.

Keep This With the House

A lifetime logbook for the house, organized by system, with room for serial numbers, install dates, repairs and contractors. It stays with the house for the next owner.

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The House Is Fifty Years Old

A buyer's guide to older homes: wiring, pipes, lead, asbestos, insurance refusals and what the repairs actually cost.